The present invention relates to an exposure device for scanning and exposing a recording medium with a light beam to engrave a desired image on a surface of a recording medium and an engraving apparatus for engraving an image in a recording medium with the exposure device.
There is known an engraving apparatus whereby a drum having a recording plate (recording medium) provided over its peripheral surface is turned in a main scan direction while an exposure device is used to scan the recording plate with a laser beam corresponding to an image to be engraved (recorded) on the recording plate in a subscan direction that is orthogonal to the main scan direction to achieve 2-dimensional scan and exposure of the recording plate with the laser beam, thereby engraving (recording) a 2-dimensional image for printing on the recording plate.
Now, where, for example, a flexographic printing plate is engraved as the recording plate mentioned above, an irradiation power of no less than several hundreds of watt will be required at the surface of the plate. To achieve this, there is proposed a multi-beam exposure system using a light source configured using low-cost high-output fiber coupled laser diodes (referred to as FC-LDs below) connected to optical fibers instead of expensive CO2 lasers or fiber lasers and optical fibers.
Where, for example, an FC-LD light source capable of irradiation power of the order of 10 watt is used, optical fibers each having a core diameter of about 105 μm will be required. However, when a laser beam having a core diameter of about 105 μm emitted from a tip of each optical fiber is caused to converge (form an image) on the plate, the converging angle (image forming numerical aperture) increases, causing such problems as increased costs for manufacturing an exposure lens (due to aberration correction) and reduction in focal depth for an exact engraving.
To avoid this, normally an aperture member is placed inside an exposure system to intercept unnecessary light. For example, an aperture member 35 is provided between a collimating lens 32 and an imaging lens 34 as illustrated in FIG. 12 or a frame member 36 of the imaging lens 34 is used instead of the aperture member as illustrated in FIG. 13 in order to block a laser beam having a numerical aperture higher than a given numerical aperture. However, this causes a problem that a large amount of heat is generated in the aperture member 35 or the frame member 36 of the imaging lens 34 owing to interception of light.
There has conventionally been made various technological proposals related to exposure devices.
U.S. Pat. No. 6,888,853 B, for example, describes a laser radiation source comprising diode-pumped fiber lasers configured that can be directly modulated as a laser beam generating source wherein output terminals of the fiber lasers are arranged parallel to each other in tracks and wherein the laser radiation beams emitted from the output terminals of the fiber lasers are collected to travel and strike a processing surface parallel to each other in their respective tracks.
JP 2004-233660 A describes an exposure device wherein light emission means and a converging lens are predisposed so that light emission units are arranged in a direction orthogonal or substantially orthogonal to an eccentric direction of the converging lens and wherein the light emission means and the converging lens are turned integrally to switch between the tilt angles of the direction in which the light emission units are arranged with respect to a preset scan direction in order to switch between resolutions at an exposure surface of lights emitted from the light emission units.
US 2006/0065147 A describes a printing plate producing apparatus by scanning a recording material by a laser beam and thus engraving the surface thereof, wherein a laser beam having a first beam diameter is used to irradiate the recording material at a first pixel pitch and thereby achieve engraving to a first depth, and subsequently a laser beam having a second beam diameter is used to irradiate the recording material at a second pixel pitch and thereby achieve engraving to a second depth.